When turbines are used on aircraft or for power generation, they are typically run at a temperature as high as possible, for maximum operating efficiency. Since high temperatures can damage the alloys used for the components, a variety of approaches have been used to raise the operating temperature of the metal components.
Nickel-base superalloys are used in many of the highest-temperature materials applications in gas turbine engines. For example, nickel-base superalloys are used to fabricate the components such as high-pressure and low-pressure gas turbine blades, vanes or nozzles, stators and shrouds. These components are subjected to extreme conditions of both stress and environmental conditions. The compositions of the nickel-base superalloys are engineered to carry the stresses imposed upon the components. Protective coatings are typically applied to the components to protect them against environmental attack by the hot, corrosive combustion gases.
A widely used protective coating is an aluminum-containing coating termed a diffusion aluminide coating. Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel and/or cobalt from the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or oxide layer that inhibits oxidation of the diffusion coating and the underlying substrate. The chemistry of the additive layer can be modified by the presence in the aluminum-containing composition of additional elements, such as platinum, chromium, silicon, rhodium, hafnium, yttrium and zirconium. Diffusion aluminide coatings containing platinum, referred to as platinum aluminide coatings, are particularly widely used on gas turbine engine components.
The second zone of a diffusion aluminide coating is formed in the surface region of the component beneath the additive layer. The diffusion zone contains various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. The intermetallics within the diffusion zone are the products of all alloying elements of the substrate and diffusion coating.
Though significant advances have been made with environmental coating materials and processes for forming such coatings, there is the inevitable requirement to repair or replace these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of the diffusion coating, refurbishment of the component on which the coating is formed, or an in-process repair of the diffusion coating or a thermal barrier coating (if present) adhered to the component by the diffusion coating. The current state-of-the-art repair process is to completely remove a diffusion aluminide coating by treatment with an acidic solution capable of interacting with and removing both the additive and diffusion layers.
Removal of the entire aluminide coating, which includes the diffusion zone, results in the removal of a portion of the substrate surface. For components, such as gas turbine engine blade and vane airfoils, removing the diffusion zone may cause alloy depletion of the substrate surface and, for air-cooled components, excessively thinned walls and drastically altered airflow characteristics to the extent that the component must be scrapped.
Most methods currently used to remove diffusion coatings to expose the surface of the superalloy component or to completely remove the additive layer include using an acid strip, multiple grit blastings, and subsequent heat tinting processes to verify that the aluminide is completely removed from the surface of the superalloy component. The acid strip uses harsh chemicals such as phosphoric, nitric, or hydrochloride acids which require special facilities to remove the additive layer and the diffusion layer.
Therefore, a method for controlled removal of at least a portion of a thickness of an additive coating from a coated superalloy component and a method for rejuvenating a coated superalloy component that do not suffer from the above drawbacks are desirable in the art.